Abstract:
Many birds can defend body temperature (Tb) far below air
temperature (Ta) during acute heat exposure, but relatively little is
known about how avian heat tolerance and evaporative cooling
capacity varies with body mass (Mb), phylogeny or ecological factors.
We determined maximum rates of evaporative heat dissipation and
thermal end points (Tb and Ta associated with thermoregulatory
failure) in three southern African ploceid passerines, the scalyfeathered
weaver (Sporopipes squamifrons, Mb≈10 g), sociable
weaver (Philetairus socius, Mb≈25 g) and white-browed sparrowweaver
(Plocepasser mahali, Mb≈40 g). Birds were exposed to a
ramped profile of progressively increasing Ta, with continuous
monitoring of behaviour and Tb used to identify the onset of severe
hyperthermia. The maximum Ta birds tolerated ranged from 48°C to
54°C, and was positively related to Mb. Values of Tb associated with
severe heat stress were in the range of 44 to 45°C. Rates of
evaporative water loss (EWL) increased rapidly when Ta exceeded
Tb, and maximum evaporative heat dissipation was equivalent to
141–222% of metabolic heat production. Fractional increases in EWL
between Ta<40°C and the highest Ta reached by each species were
10.8 (S. squamifrons), 18.4 (P. socius) and 16.0 (P. mahali). Resting
metabolic rates increased more gradually with Ta than expected,
probably reflecting the very low chamber humidity values we
maintained. Our data suggest that, within a taxon, larger species
can tolerate higher Ta during acute heat stress.
